Accelerated Discovery of New 8-Electron Half-Heusler Compounds as

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C: Energy Conversion and Storage; Energy and Charge Transport

Accelerated Discovery of New 8-Electron Half-Heusler Compounds as Promising Energy and Topological Quantum Materials * Vikram, Bhawna Sahni, Chanchal Kumar Barman, and Aftab Alam J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01737 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 11, 2019

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The Journal of Physical Chemistry

Accelerated discovery of new 8-electron half-Heusler compounds as promising energy and topological quantum materials Vikram, Bhawna Sahni, C. K. Barman and Aftab Alam∗ Department of Physics, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India E-mail: [email protected]

Abstract Rapid discovery of potential functional materials remains an open challenge. We often focus on exploring the properties of previously reported compounds, but avoid various unreported but chemically plausible compounds that might have promising properties. Here, we present a high throughput ab-initio study of I-III-IV class of half-Heusler alloys with 8-valence electrons, in the quest of finding potential (i) thermoelectric (ii) topological insulating and (iii) opto-electronic materials. Of various class, 8-electron half-Heusler compounds are least studied, and hence our choice. By carefully choosing reliable and accurately simulated descriptors (such as formation energy, phonon dispersion, accurate bandgaps), we have discovered 21 semiconducting compounds. Out of these 21 compounds, 6 were found to show excellent thermoelectric performance (figure of merit ZT> 0.8), other range from ZT= 0.2 to 0.8. 11 compounds were found to show robust topological insulating behavior confirmed by bulk band inversion, and surface conducting states. Two compounds show relatively

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large band gap and can be promoted for possible optoelectronic applications with further band engineering. Our search model opens new avenues for the discovery of more novel materials in different and unexplored class of systems. We strongly propose the experimental characterization of the above promising compounds to shed more light on the present findings.

February 1, 2019

1

Introduction

Since the discovery of Heusler alloys in 1903, they have been the center of diverse scientific interests. Half-Heusler (HH) compounds, also called ternary Heusler alloys, have a general composition of XYZ where generally X, and Y are transition metals or metals (in general) and Z is a p-block element. The qualities, such as easy synthesis of intrinsic defect free samples with wide varieties of choices for the three sites (X,Y and Z), give rise to the vast plethora of Half Heusler compounds. These choices of numerous elements make the band gap, band topology and other material properties to be easily tunable which makes this class of compounds suitable for useful applications such as spintronics, non magnetic semiconductors, optoelectronics, thermoelectrics, topoloical insulators (TIs), superconductors, etc. 1 HH compounds are generally classified according to the total sum of the valance electrons present in X, Y and Z elements. HH compounds with valance electron count (VEC) 18 are most widely studied from both experimental and theoretical aspects. 2–6 In fact, R. Gautier et al. 7 in 2015 published an article in nature chemistry, reporting all the possible compounds which were previously unknown with VEC 18, suggesting various possible scientific applications like optoelectronics, thermoelectrics etc. The quest of improving the material properties for a specific application can be achieved from two fronts, (i) suitably doping the existing state-of-the-art materials, and (ii) finding new compounds with better properties. 8 VEC HH alloys are best suited for the second aspect as they are not much explored. 8-electron

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HH alloys can be realized in various configurations such as, I-I-VI, I-II-V, I-III-IV, II-II-IV and II-III-III, where each of the number in roman letters are the valance electrons in X, Y and Z elements, respectively. Most widely studied class of 8-electron HH alloys are the I-II-V compounds. 8–18 Some of these compounds are found to be promising for solar cell applications. In the past, people have also performed first principles high throughput calculations to study the elastic, 11 optical, 16,17 piezoelectric 19 and even topological insulator study 20 of I-II-V and I-III-IV class of materials. Although, some of these studies are robust, the search for topological insulators and optoelectronic properties require accurate prediction of band gaps which is ignored in these studies. For instance, Wang et al. 20 proposed few 8 VEC half Heusler compounds to show topological insulating properties. These compounds, however, turned out to be trivial semiconductors with no band inversion in our study, when a more accurate/robust Hybrid functional calculations were performed. This emphasizes the necessity of using accurate exchange-correlation functional to correctly predict the governing properties for any application. Although, there is a great scope to discover new materials in this class which are promising for thermoelectric applications, none of the existing literature illustrates this possibility. Only a few compounds in I-III-IV class, namely, KScX(X=C, Ge) 21 and LiAl(Si,Ge) 22 are explored for thermoelectric application but are still incomplete and lack a detailed in-depth study. With the advanced predictability power of first principles electronic-structure codes, we have reached a stage to accurately predict unknown thermodynamically stable compounds as well as additional predictions of the functionalities of these materials. In this paper, we have systematically evaluated and presented a detailed high-throughput first-principles study of electronic structure, phonon, electrical as well as thermal transport properties of I-III-IV class of 8-electron HH alloys without compromising the convergence criteria and the accuracy of the calculations. The key idea is to explore the possibility of discovering new materials with potential applications in thermoelectrics, topological insulators (TIs) and optoelectronics. We have used various screening parameters/markers like formation

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energy, phonon frequencies, accurate band gap values (explained later), nature of band gap (direct/indirect), thermoelectric figure of merit (ZT), and topological band inversion to converge the search for suitable candidates. Detailed calculations of most promising candidate materials for specific applications are then performed. Careful consideration of the lattice thermal conductivity and the bipolar thermal conductivity has been done for narrowing down the potential candidates with superior thermoelectric performance.

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Computational Details

Ab-initio calculations were performed using density functional theory (DFT) 23 as implemented in Vienna Ab initio simulation package (VASP) 24–26 with a projected augmentedwave basis 27 and the generalized gradient approximation (GGA) exchange-correlation functional of Perdew-Burke-Ernzerhof (PBE). 28 For more reliable estimates of the band gap, hybrid functional (HSE06) 29 was used for all the systems. Single shot GW (G0W0) calculations were also performed on top of HSE06 to further improve the band gap of the compounds. A plane wave cut-off of 500 eV was used. The Brillouin zone sampling was done by using Γ-centered k-mesh. For all the compounds, k-point mesh of 10×10×10 (ionic relaxations) and 20×20×20 (self-consistent-field solutions) were used for PBE calculations. A 10×10×10 k-mesh was used for hybrid (HSE06) and G0W0 calculations. As most of the compounds are made up of light elements, spin orbit coupling (SOC) was not included for transport calculations. SOC was only included for careful evaluation of the band inversion strength (BIS) to predict accurate topological insulator behavior of the compounds. Cell volume, shape and atomic positions for all the structures were fully relaxed using conjugate gradient algorithm till the energy (forces) converges to 10−6 eV (0.001eV /˚ A). Tetrahedron method with Blochl corrections 30 was used for accurate electronic density of states. All the compounds (XYZ) were considered to crystallize in F-43m (#216) space-group with X at 4a(0,0,0), Y at 4c(0.25, 0.25, 0.25) and Z at 4b(0.5, 0.5 0.5) Wyckoff positions.

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The formation energy (∆Ef ) was calculated as,

∆Ef = E(XY Z) − [E(X) + E(Y ) + E(Z)]

(1)

The semi-classical Boltzmann transport formalism as implemented in BoltzTraP code 31 was used to calculate the TE properties of selected candidates. Debye-Callaway model 32 was used for lattice thermal conductivity calculations. Density functional perturbation theory (DFPT) combined with Phonopy 33 was used to obtain relevant phonon properties. Further computational methods/details are explained in Sec. I, of the supplementary material. 34

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Results and Discussion

Figure 1 shows all the possible choices of elements from periodic table, which can form I-IIIIV class of 8-electron XYZ HH alloys, which will yield 320 compounds. Group IVB elements cannot be taken, because the Z-elements need to be a p-block element for the alloy to form. Further, keeping in mind the ease of synthesis along with cost effectiveness and abundance of the elements involved, we have discarded elements which are either expensive, toxic, rare or dangerous. This leaves us with X = Li,Na, K, and Cu; Y = B, Al, Ga and In; and Z = C, Si, Ge, and Sn; reducing the number of total compounds to be 64. However, for a given compound XYZ, there are three unique possibilities for the site preference of the X, Y and Z elements at three wyckoff sites (as shown in Fig. 1 (top panel)). This leaves us with a total of 64 × 3 = 192 possible compounds to be studied. In order to further screen these compounds for specific applications, various markers like formation energy (chemical stability), phonon frequencies (thermal stability), accurate band gap values (PBE+HSE06+G0W0), nature of band gap (direct/indirect), thermoelectric figure of merit (ZT), and band inversion were used.

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Elements Excluded Toxic (Pb, Tl) Expensive (Ag, Au) Dangerous (Rb, Cs) Rare (Sc, Y, La) XYZ X= Li, Na, K, Cu Y= B, Al, Ga, In Z= C, Si, Ge, Sn 64 compounds

XYZ ZYX

XYZ

XZY YZX

XZY

YXZ ZXY

YXZ

192 Compounds

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 1: Periodic table of elements showing I-III-IV class of elements (highlighted) used in the present study. Inset at the top shows the elements used and their possible combinations to study site preference to ensure structural stability.

3.1

Structural, Chemical and Thermal Stability

The initial screening of 192 compounds is based on the chemical and thermal stability. Figure 2 shows the formation energy of all the 192 compounds as a function of the average atomic number (also see Sec. II A of supplement, SI 34 for more details). The stable (unstable) compounds with negative (positive) formation energy are shown with green circles (red squares). 50 out of 192 compounds were found to have negative formation energy, indicating the chemical stability of these compounds. Inset of Fig. 2 shows the formation energy of the stable compounds only. It is interesting to note that few compounds have all the three configurations (e.g. LiAlSn, LiSnAl, and AlLiSn) to be quite stable with negative formation energy, although one of them is most favored. This may indicate the presence of more than one stable phases or small anti-site disorder in these compounds. We have considered all such stable phases for further calculations. We have then simulated the phonon dispersion for all these 50 compounds. All these compounds confirm their dynamic stability except 4 namely AlLiGe, GaLiGe, LiInGe, and CuGeAl which showed imaginary frequencies. These compounds are shown with yellow diamonds in Fig. 2 (see Sec. II A, SI 34 for details). Thus, a total of 46/192 compounds were found to be both chemically and thermally stable. 6


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∆Ef (meV/atom)

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∆Ef (meV/atom)

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Figure 2: Formation energy (∆Ef ) of 192 compounds as a function of their average atomic number. Negative (positive) values signify the stability (instability) of the compounds and is depicted by green circles (red squares). Compounds which have negative formation energy but imaginary phonon frequencies are shown with yellow diamonds. Inset shows enlarged view of the stable compounds.

3.2

Band gap

After shortlisting 46 stable compounds, we proceeded with the bandgap calculation which is our next descriptor. Figure 3 shows the simulated bandgaps and the optimized lattice parameter for these 46 compounds (more details in Table SII, SI 34 ). PBE exchange correlation functional is known to underestimate the band gap. This apparently causes most of the compounds to reflect metallic behavior. A total of 16 compounds were found to show semi-conducting band gaps with PBE. This number increases to 18 when single-shot G0W0 calculations were performed. Hybrid functionals (HSE06) are known to better estimate the band gaps, which can further be improved by G0W0 calculations. We have performed HSE06 and HSE06+G0W0 calculations for better prediction of the band gaps. G0W0 calculations were found to enhance the HSE06 band gaps and a total of 21 compounds were found to show significant band gaps (ranging from 0.02 to 1.88 eV) with HSE06+G0W0. These various exchange correlation functional are found to have negligibly small effect on the band topology near the band edges, and thus, only changes in the band gap are discussed here.

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Symbol description in Fig. 3 goes as follows: lattice parameter (star; black), and for band gaps, PBE (circle; orange), PBE+G0W0 (square; red), PBE+HSE06 (diamond; green) and PBE+HSE06+G0W0 (traingle up; blue). The most accurate PBE+HSE06+G0W0 band gaps for these 21 compounds were then used for further calculations. 1.8

6.8

1.6

6.6 6.4

1.4 PBE PBE+G0W0 PBE+HSE06 PBE+HSE06+G0W0 Lattice constant

1.2 1 0.8 0.6

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0.4

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0.2

5

0

Lattice constant (Å)

2

Band gap (eV)

4.8 LiBSi LiBGe LiCAl LiAlSi LiSiAl LiAlGe LiGeAl LiAlSn LiSnAl LiGaSi LiSiGa LiGaGe LiGeGa LiGaSn LiSiIn LiGeIn LiSnIn NaAlGe NaGeAl NaAlSn NaSnAl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 3: Band gaps and lattice parameter of 21 compounds found to be semi-conducting. The band gap values are calculated at PBE (circle; orange), PBE+G0W0 (square; red), PBE+HSE06 (diamond; green) and PBE+HSE06+G0W0 (triangle up; blue) level. The absence of data for any given compound corresponding to a particular exchange correlation functional implies its metallic behaviour. PBE+HSE06+G0W0 band gap values are our most accurate estimate and are used for further calculations. It is interesting to note that all the band gap values lie below 1 eV, which is desirable for both TE and TI applications, except LiCAl (1.88 eV) and LiAlSi (1.03 eV). The latter two compounds can be promoted as solar harvesting material via some band engineering which can bring the band gap in the visible range. Henceforth, we will discuss about the possible applications of above compounds as thermoelectric material and topological insulator.

3.3

Further screening

Thermoelectric properties of 21 compounds were calculated to further short-list the most promising candidates for TE applications. The semi-classical Boltzmann transport formalism as implemented in BoltzTraP code 31 was used to calculate the TE properties of selected 8


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candidates. Debye-Callaway (D-C) model was used to calculate the lattice thermal conductivity (κL ) for all the compounds. For cross validation of D-C model, the calculated κL for a well-studied system ZrNiSn is compared with existing experimental data and also a more accurate theoretical yet computationally expensive data. The details of D-C model and the comparison of lattice thermal conductivity for ZrNiSn are shown in Section I(B) of supplement. 34 This comparison facilitated us with a higher confidence on the accuracy of the D-C model for 21 unknown compounds, studied in this manuscript. Six out of 21 compounds were found to give excellent TE properties with ZT>0.8. Table 1 shows the Seebeck coefficient (S), power factor (S 2 σ), ZT, optimal carrier concentration (n) for both n- and p-type conduction for these six compounds. The rest 15 were found to give ZT>0.5. The ZTmax obtained for both n and p-type conduction and the corresponding TE properties for all the compounds are listed in Table SIII of supplement. 34 Also, the electronic band structure, phonon dispersion, Gruneisen parameter and TE properties (S, S2 σ and ZT) of 20 systems are shown in Fig. SIII to Fig. SXXII of supplement. 34 We choose to discuss the TE properties of the most promising compound (LiAlGe) in some detail, in the following section.

3.4 3.4.1

Thermoelectric application : LiAlGe Electronic structure

LiAlGe is found to be chemically stable with a formation energy of -806.63 meV/cell. It has ˚ which is comparable to the a perfect cubic lattice with a optimized lattice constant of 6.06 A ˚ 22 The most accurately simulated bandgap obtained experimentally reported value of 5.98 A. using HSE06+G0W0 exchange correlation is 0.78 eV, which is used in further transport calculations. Fig. 4(a) shows the atom projected electronic band structure and the density of states for LiAlGe. The valance band edges are mostly comprised of Ge with minor contributions from Al and Li. The conduction band edges have significant contributions from all the three 9



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Table 1: The maximum ZT values, and the corresponding Seebeck coefficient (S, µVK−1 ), Power factor (S 2 σ, mWm−1 K−2 ), carrier concentration (n, ×1019 cm−3 ) and the temperature (T, K) for both n and p-type conduction. These are the only selected candidates whose maximum ZT value is >0.8, either for n or p-type conduction. S.No. Compound Doping S LiAlSn n 262.45 1 p 266.37 LiSiGa n 243.34 2 p 272.29 LiSiIn n 287.49 3 p 291.35 NaAlGe n 307.31 4 p 319.06 NaAlSn n 268.98 5 p 234.36 LiAlGe n 345.01 6 p 374.68

S 2σ ZTmax 9.28 0.82 5.15 0.65 11.94 0.79 4.57 0.52 9.39 0.87 3.92 0.54 4.75 0.86 2.96 0.75 7.35 0.87 4.01 0.58 6.05 0.83 4.77 0.81

n 24.32 5.12 27.47 3.75 14.90 3.69 3.15 3.64 8.71 8.31 8.02 2.81

T 900 860 900 900 900 900 900 900 900 900 900 900

atoms along Γ to L, but the edges along Γ to X, X to W, and W to L have major contribution from Li only. The relatively flat nature of bands along Γ to L direction for both conduction and valance band edges indicates higher effective mass values which is a desirable feature for better thermoelectric performance.

3.4.2

Phonon properties

Figure 4(b) shows the phonon dispersion for LiAlGe along high symmetry directions. The acoustic and optical modes of vibration are shown by green and black lines respectively. Since the primitive unit cell has three atoms, there are a total of 9 phonon branches (3 acoustics and 6 optical). The acoustic modes are further classified into one Longitudinal acoustic (LA) and two transverse acoustic modes (TA and TA0 ). The thermal conduction is mainly due to the acoustic modes of vibration. Notably, the two TA and TA’ bands are degenerate along Γ to X but they split along Γ to K direction. The velocity of each acoustic mode (ν) is given by the slope of the band corresponding to the vibrational mode at Γ point. Debye temperature (θ) can be obtained from the maximum frequency corresponding to the

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Frequency ν (THz)

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Figure 4: For LiAlGe, (a) atom projected electronic band structure and the corresponding density of states (Fermi level at 0 eV), (b) phonon band dispersion, (c) variation of mode gruneisen parameter γi with phonon frequency, (d) temperature dependence of lattice and bipolar thermal conductivity, and (e) variation of important TE properties (S, S2 σ, and ZT) with chemical potential (µ) (left) and temperature (right). The temperature dependent TE properties are calculated for a carrier concentration of 8.02×1019 cm−3 and 2.81×1019 cm−3 for n and p-type conduction, respectively.

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vibration mode. In addition to ν and θ, the D-C model for the calculation of lattice thermal conductivity also requires mode gruneisen parameters, γi (see Sec. IB of SM 34 ). Figure 4(c) shows the phonon frequency dependence of gruneisen parameters. Maximum value of γi for each vibrational mode is taken for the lattice conductivity calculation in the D-C model. The values of all the parameters used to calculate (κL ) for LiAlGe are, νLA =5787.18 m/s, νT A =2182.72 m/s, νT A0 =530.32 m/s, γLA =2.06, γT A =1.97, γT A0 =3, γ=3.33, θLA =238.65 K, θT A =183.06 K, θT A0 =90.35 K, V=18.57 × 10−30 m3 and M=58.96 × 10−27 Kg. The lattice thermal conductivity (κL ) obtained from the D-C model is shown in Fig. 4(d). The simulated κL of 3.27-2.72 Wm−1 K−1 is in good agreement with the experimental value of ∼ 2.9 Wm−1 K−1 in temperature range 300-400 K. 22 Notably, κL varies from 2.99 to 0.5 Wm−1 K−1 in the temperature range 300-900 K. It is important to note that the minority charge carriers start to become important when the temperature is sufficiently high which generates electron-hole pair and the excitation of electrons across the band gap. The thermal excitation due to such a process introduces an additional component to κ, called the bipolar thermal conductivity (κb ). Figure 4(d) shows the temperature dependence of κb for both n and p type carriers. κb plays an important role in the total thermal conductivity beyond ∼450 K, as evident from Fig. 4(d).

3.4.3

Thermoelectric Properties

Figure 4(e) shows the chemical potential (µ) and temperature dependence of simulated Seebeck coefficient (S), power factor (S2 σ) and ZT for both n and p-type conduction. The temperature dependent TE properties were calculated for a carrier concentration of 8.02×1019 cm−3 and 2.81×1019 cm−3 for n and p-type carrier respectively. At these carrier concentrations, the maximum Seebeck coefficient and power factor obtained for n-type conduction are 345.02 µVK−1 and 6.06 mWm−1 K−2 , and for p-type conduction are 374.68 µVK−1 and 4.77 mWm−1 K−2 , respectively. Compared to the most promising TE materials existing in the literature (undoped), significantly high ZT values (0.83 and 0.81 for n and

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p-type conduction respectively) were obtained for LiAlGe.

1.5

LiGaSi LiGeAl LiSiGa LiAlSn

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8 6

LiGaGe LiGeGa

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Topological Insulators: Eight valence electron Heusler alloy

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Figure 5: (a) The band inversion strength, BIS (EΓ6 - EΓ8 ) vs optimized lattice constants (aopt ) for all the compounds, (O and × symbols are used to distinguish the nearby compounds) (b) HSE06+SOC bulk band structure of LiGeIn at ambient lattice parameter (a = 5.61˚ A), (c) surface bands for LiGeIn (red circles and green squares represent the contribution of surface atoms from the bottom three (X-Y-Z) and top three (Y-Z-X) atomic layers respectively) and, (d) unit cell for the (111) surface. We further dig into the 46 stable half-Heusler compounds and found some of the promising candidates to host topologically non-trivial band topology. The non-trivial band topology requires inverted band order between s-like (Γ6 band) and p-like (Γ8 band) orbitals at Γ point. The measure of non-triviality is defined by band inversion strength (BIS), which is the energy difference between s-like Γ6 and p-like Γ8 bands at Γ point. Negative value of BIS indicates non-trivial band order while positive values indicate trivial band order. Based on HSE06+SOC calculations, we have identified 17 compounds at ambient lattice, to show inverted band order. Figure 5(a) shows the BIS (EΓ6 - EΓ8 ) vs optimized lattice constants (aopt ) for all these compounds. Interestingly, some of these compounds turn out be semimetal (with zero band gap) and metal along with non-trivial band order as described in Table SII in supplement. 34 Moreover, we also found few compounds whose BISs are positive but lie close to the zero BIS line. Such compounds are likely to invert their band order under small perturbation such as pressure, strain, or doping. Larger the value of −ve BIS, more robust the non-trivial properties are. In fact, few of these predicted topological semi(metals) (TSM) 13


Γ


show quite large BIS (upto -2.16 eV), as compared to other state of the art TI materials. 35,36 Next, we choose one of the most promising TSM candidate, LiGeIn and extend a detailed analysis of its topological properties. Figure 5(b) shows the bulk band structure of LiGeIn at ambient lattice constant (a = 5.61 ˚ A). Clearly, this system is a zero gap semi-metal with an inverted band order. The p-like bands preserve the four fold degeneracy at the Γ-point while the s-like states (shown by red dots) lie, in energy, below the p-type states owing to the band inversion. Surface bands are important to clarify the topological class of a material. Ternary halfHeusler compounds exhibit a layered structure along [111] direction and thus the most naturally cleavable surface is the (111) plane. In order to investigate the surface bands, a surface slab along [111] direction is constructed for LiGeIn at aopt . A 36 atomic-layer thick slab, with Lithium (Li) atomic termination at both bottom and top surface, is simulated for the surface band structure as shown in Fig. 5(c). A vacuum of 15 ˚ A is introduced to minimize the coupling between top and bottom atomic-layers. Five atomic layers on each of the top and bottom sides of the slab are fully relaxed, such that the force (energy) is converged up to 0.001 eV/˚ A (10-6 eV). We have also simulated slabs with different possible atomic terminations. However, the slab with Li-termination at both ends turn out to be energetically the most favorable, as discussed in more detail in the supplement (see table SIV 34 ). Projection of the surface bands are also shown in Fig. 5(c). The size of red circles and green squares represent the weighted contributions of surface atoms from the bottom three (Li-In-Ge) and top three (Li-Ge-In) consecutive atomic layers respectively (see the surface slab structure in Fig. 5(d)). The contributions on these surface bands from other deeper atomic layers (from fourth atomic layer onwards) are significantly small. Figure 5(c) clearly shows the metallic nature of the surface conduction governed by surface mediated bands. A similar nature of surface bands have also been experimentally observed in ternary half-Heusler alloy RPtBi. 37 ¯M ¯ line segment One can also notice an odd number (total #11) of Fermi crossings along Γwhich is directly consistent with the strong topological insulating phenomenon. 36,38

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4

Conclusion

We have performed a systematic search of missing 8-valance electron half-Heusler materials using first principles calculation. Such high throughput approach holds several advantages: (i) It paves a path as to which of the previously overlooked compounds can possibly be stable in a given structure with specific properties. (ii) It helps to narrow down the set of materials that need to be focused for experimental characterization for given functionalities (thermoelectric, topological insulators etc.). This saves tremendous efforts, time and energy of experimentalists that could have wasted by targeting unstable compounds. In this article, our research focuses on I-III-IV class of 8-electron half-Heusler compounds to find promising candidates for thermoelectric, topological insulator and opto-electronic applications. 46 compounds were found to be both chemically and dynamically stable. An accurate band gap calculations qualifies 21/46 compounds to be semiconducting. 6 out of 21 compounds were found to show high thermoelectric figure of merit (ZT> 0.8). 17 compounds were found to show topological non-trivial properties with robust surface conduction. These predictions are made for pristine samples, and hence there exists plenty of rooms to enhance their properties via different routes such as doping, pressure, nano-composites etc. We strongly believe that such computational efforts can direct the experimentalists towards realistic targets and speed up the discovery of new novel materials. It is highly desirable to experimentally synthesize the proposed compounds here and verify their functional properties.

Supporting Information Supporting Information Available: (I) Computational methods, (II) Screening data (a) Structural and thermal stability, (b) Theoretical Band gaps, optimized lattice parameters and topological nature, (c) Thermoelectric properties, and (d) Topological Insulator: LiGeIn. This material is available free of charge via the Internet at http://pubs.acs.org 15



Author Contributions AA and V came up with the fundamental idea behind this project. V performed all the initial screening calculations along with the calculation of thermoelectric properties, results of which, are presented in the manuscript. BS performed the detailed calculations of the potential TIs presented in the manuscript. CKB helped in analysing the TI results and editing of the paper. Furthermore, V wrote the initial draft of the paper. AA directed the analysis of results and editing of the paper.

Competing financial interests The authors declare no competing financial interests.

Acknowledgment Vikram acknowledges financial support from Indian Institute of Technology, Bombay in the form of teaching assistantship. AA acknowledges National Centre for Photovoltaic Research and Education (NCPRE) (financially supported by Ministry of new renewable energy (MNRE), Government of India) to support this research.

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Accelerated Discovery of New 8-Electron Half-Heusler Compounds as

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